1. Field of the Invention
The present invention relates to a wire dot print head in which print wires are driven to advance and withdraw to thereby perform impact dot printing.
Wire dot print heads are capable of high-speed printing and superior in cost performance and hence widely used in serial printers, line printers, etc.
2. Description of the Related Art
In a typical wire dot print head, a distal end guide is provided to guide the distal end of each print wire, which is driven to advance and withdraw, so as to prevent it from deviating from a predetermined position.
However, as the length of the print wires increases, the above-described wire guide structure becomes unsatisfactory. That is, as each print wire is driven to advance and withdraw, the print wire vibrates with nodes at the proximal end fixing portion and the distal end guide portion. The amplitude of the vibration increases as the print wire lengthens, so that the print wire is likely to break in the vicinity of the distal end guide due to fatigue.
Therefore, it has been conventional practice to provide an intermediate guide for guiding the print wire so as to prevent it from oscillating in the diametrical direction at a position corresponding to the antinode (loop) in the primary natural vibration mode of the print wire, that is, a position in the middle between the proximal end fixing portion and the distal end guide portion.
If the intermediate guide is thick, it may interfere with the movement of the print wires, which are driven to advance and withdraw while warping to a certain extent. Therefore, the intermediate guide is formed by disposing two thin plate members at a spacing, which are provided with guide holes through which the print wires pass loosely. If the intermediate guide comprises a single plate member, the pressure that is applied by the print wires concentrates thereon. Accordingly, the intermediate guide is likely to be damaged or deformed. If three or more plate members are used, one or more of them fail to contact a print wire. Therefore, there is no sense in increasing the number of plate members more than two.
However, even if an intermediate guide is provided in the vicinity of the antinode in the primary natural vibration mode of the print wires to prevent oscillation thereof, as described above, the accident that a print wire is broken in the vicinity of the distal end guide portion has still occurred at a considerably high rate.
The present inventors have noticed that the above-described accident is caused by the secondary natural vibration of the print wires.
FIG. 21 shows distributions of bending stresses (C) generated in a print wire 9 in two cases: (A) where the print wire 9 vibrates in the primary natural vibration mode, and (B) where it vibrates in the secondary natural vibration mode.
As will be clear from the figure, for the same maximum amplitude, the stress that is generated in the print wire 9 in the vicinity of a distal end guide 3 in the secondary natural vibration mode is about 2.5 times larger than that in the primary natural vibration mode. Accordingly, even if the primary natural vibration is successfully suppressed, the print wire 9 is likely to break due to the stress generated by the secondary natural vibration.
Incidentally, the intermediate guide, which guides the intermediate portions of the print wires, is formed from a plastic material. However, since it is formed in a thin plate-like shape, the intermediate guide is readily warped. In addition, since the contact pressure of the print wires is high, the intermediate guide is likely to wear at a high rate. As the wear progresses, the guide holes enlarge, which makes the intermediate guide unable to function as a guide.